Allyllithium compounds

In particular 1- or 3-hetero-substituted allylzinc reagents are often prepared in situ by adding anhydrous zinc dichloride or dibromide to the solution of the allyllithium compound. 

The high nucleophilicity of heterosubstituted allyllithium compounds makes them attractive reagents in synthetic organic chemistry. Structural studies of these compounds give a fundamental understanding about the control of the regioselectivity. Often, these studies are difficult due to the tendency of the compounds to form complex fluxional aggregates in solution. Piffl and coworkers have studied the dependency of the oxidation state of sulfur on the structure and electronic properties of the heterosubstituted... 

In contrast, sodium p -nitrophenoxide is similar to the allyllithium compounds. The counter cation reduces the p character of the oxygen and prevents the delocalization of the negative charge over the aromatic system. The valence orbitals are centered toward Na+, whereas for the naked anion they are more disposable or, in other words, more diffuse (Scheme 22). Therefore, in the latter case, unfavorable out-of-phase overlap with the leaving group is increased, and rear-side attack of the nucleophile is promoted, leading to inversion (Table XI). 

Some examples which show the dynamic effects of alkyl and silyl substituents on barriers to rotation in allyllithium compounds 26 to 32 are listed in Table 7. These results were obtained from proton NMR line shape data. The procedure for compounds 26, 28, 29 and 32, which exhibited rotation around their C — CH2 bonds, is diagrammed by structure 33 in which hydrogens A, B and X are all nonequivalent and each couples to the others. Rotation averages the A and B shifts as well as the coupling constant between them and 3./(Ha.Hx) averages with 37(HB,HX), (Figure 15). At the same time, the Hx resonance... 

Inspection of Table 7 shows that rates of rotation in allyllithium compound 32 and the associated barriers depend strongly on the nature of lithium solvation34. Thus the rotational process cannot involve a free allylic anion alone. Rather, the process must be driven by a chemical mechanism. This has been proposed by calculations to involve the development of a degree of Li—C (terminal) covalence in the transition state accompanied by some change in the solvation around lithium. It is demonstrated for compound 32 in Figure 17. 

Allylalkali metal compounds exhibit dynamic isomer-izations and bond rotations in solntion. In THF solntion, allylalkali metal complexes undergo C bond rotations with activation energies normally in the range of 45-75kJmoL. 1 - AUcylallylalkali metal compounds usually exist as a dynamic mixture of (E) and (Z) isomers (15), with a thermodynamic preference for the (Z) configuration. The rates of isomerization of frani -neopentylallyl metal complexes in THF solution decreases in the sequence Li > Na > K. Numerous studies of the structures and dynamic properties of allyllithium compounds have been reported. " ... 

Allylstaimanes are available from reactions of allybnagne-sium or allyllithium compounds with tin halides (equation 32). Other syntheses involve reactions of triorganotin anionic species with allylic halides or esters (equation 14), as well as reactions of allylic xanthates, tosylates, etc. with tin hydrides (equation 33). 

To explain these changes in selectivity, a reaction scheme for allyllithium-catalyzed butadiene polymerization is formulated (Scheme 4), which is derived tentatively on the basis of experimental and theoretical investigations of the structure and reactivity of the allyllithium compounds. 

The cyclopropyl group shows its customary pseudo-vinyl behaviour (entry 3), and, as with the vinyl compounds, the transmetallations take place with retention of configuration.15 Allyl compounds (entry 5), on the other hand, lose their structural identity (e.g. equation 22-4) because of the symmetrical structure of the allyllithium compounds.12-21... 

Siloxy-substituted Allyllithium Compounds via [1,2]-Brook Rearrangement... 

This test showed that a-amino- [7] and a-oxy- [8] alkyllithium compounds except for their benzyl- or allyllithium compounds are configurationally highly stable, and a-thio-[9], a-selenoalkyllithium compounds [10], and lithiated phosphine oxides [11] are labile. Thus, this test proves that lithium carbanions are either configurationally stable or labile and affords reliable evidence to de-... 

Allyltitanium reagents have been used in the homo-aldol reaction [76]. Enan-tiodivergent tuning by virtue of the titanium reagents of chiral l-oxyallyUithium substrates leads to enantiomeric homo-aldol products with excellent enantiomeric excess. Hoppe and coworkers found the first example of a chiral nonracemic allyllithium with configurative stabiHty [77]. In their work they described the lithiation of optically pure aUyl carbamates. These lithiated allylic carbamates are substrates for homo-aldol reactions. The so-derived chiral allyllithium compounds can be transmetaUated to titanium in order to facilitate a subsequent aldol reaction (Scheme 3.48) [76a]. 

Functionalized allyllithium compounds of type XIII are also homoenolate equivalents [122,130], but in their reaction with electrophiles sometimes it is not possible to control the regioselectivity. These compounds have been prepared mainly by either deprotonation or tin-lithium exchange. Deprotonation of (F)-cinnamyl-N,N-diisopropylcarbamate 155 with n-BuLi in the presence of (-[-sparteine in toluene gave a configurationally stable lithiated O-allyl carbamate (epi-156), which equilibrates at -50 °C to give the (R)-intermediate 156. Whereas the reaction of these compounds with Mel and MeOTs gave the /-attack, however acylation, silylation and stannylation took place at the a-position (Scheme 2.21) [131]. 

The allyllithium compound 227 was generated from the phenylsulfanyl precursors by means of lithium naphthalenide in THE at -78 °C in the presence of TMEDA and underwent a irreversible retro- [l,4]-Brook rearrangement to give an almost 4 1 mixture of syn,trans and anti.trans diastereomers [186]. 

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